In the vestibular system of the inner ear, motion is detected via the mechanical deflection of a bundle of stereocilia located at the top of sensory receptor hair cells. The bundle is morphologically and physiologically polarized because only movements of the bundle towards a lone kinocilium positioned at one side of the apical cell surface are able to produce excitatory responses. Thus the range of motion that can be detected by an individual hair cell is determined by the polarized orientation of the stereocilia bundle. As a result, in order to respond to the broadest range of motions the utricle and saccule contain thousands of vestibular hair cells arranged in radiating arrays spanning a range of nearly 360ps of stereocilia bundle orientations. This is achieved in part by dividing the hair cell between two groups divided by a Line of Polarity Reversal (LPR) that have opposing stereocilia bundle orientations and respond to motions in opposite directions. Our goal is to identify the genetic mechanisms that direct the development of planar polarity. This will be addressed through the course of the project using combinations of knockout and transgenic mouse models. Specifically we will test the hypothesis that the core Planar Cell Polarity (PCP) proteins establis an underlying ground polarity that coordinates the orientation of adjacent hair cells regardless of their position relative to the LPR, and that a second patterning mechanism positions the LPR. For these experiments the significance of the PCP-based ground polarity will be established using a combination of single and double knockouts mouse lines that prevent core PCP signaling. This will complement a genetic dissection of Wnt-signaling and its parallel role in positioning the LPR. Finally, additional factors directing formation of the LPR will be identified through genetic labeling and FACS-based isolation of hair cells with opposite bundle orientations followed by microarray analysis. Although focused on the development of vestibular planar polarity, we anticipate that this research will impact our understanding of auditory planar polarity as well as other organ systems that rely upon cellular polarization for growth or function